Specify "Unrestricted" for calculation with odd number of electrons

Dear all,

I have, I think, a very simple question. It’s basically in the title.
I just want to do a few opt, freq and tddft calculations on complexes in different oxidation states and wanted to know if I have to specifically set “unrestricted = 1” for the forms with an odd number of electrons such as radical cations (1 2 in the $molecule section).

Thank you and sorry if this has been answered before.

Short answer:
for DFT and TDDFT using Unrestricted=TRUE is recommended on the basis of the theoretical arguments by Pople and others. However, you do need to monitor spin-contamination of the resulting states.

The issue of spin-contamination in open-shell calculations is discussed in this tutorial chapter, which you may find helpful: http://iopenshell.usc.edu/pubs/pdf/revcc-30.4-151.pdf

Thank you very much for the swift answer and the extra tutorial material!

For any multiplicity other than singlet, the SCF calculation will default to unrestricted any you don’t have to specify anything.